Update trim values for sample on sample cache update

[Ben Wiley]

When post-plugin duration changes (before-pitch shift duration), it would be ideal to somehow trigger an update to trim frames, based on the ratio of the new to the old duration, before computing the rest of the audio. For this we might want to split the function into two, and provide a way for the SampleCache update to spit out a modified SampleContainer which the caller should save (without generating yet another SampleCache).